Types with default value of `null` don't list `null` as a valid type.

[chris-muller]

I'm trying to use the json-schema-to-zod npm package to generate Zod schemas for these schema but there's a lot of properties listed that are marked as having a default value of null but only have "type": "string" instead of "type": ["string", "null"].

For specific examples you can look at harmonized_tariff_code and country_of_origin in create_shipments_request_body.json.

This is causing an issue with the json-schema-to-zod package since the generated Zod schema gives a type error: Argument of type 'null' is not assignable to parameter of type 'string'.

country_of_origin: z
  .string()
  .min(2)
  .max(2)
  .default(null),

It would be really helpful if the nullable properties were updated to include null in their allowed types (or if they aren't actually nullable, then remove the default values).